Make an overlay always on top of other overlays

#### Please describe the feature you would like to request.

whenever a new overlay is created it is positions on top of other existing overlays (speaking wrt z-index).
provide a way to mark a overlay need to be shown always on top of other overlay's irrespective of order of creation

#### What is the use-case or motivation for this proposal?

In our application we are using `overlay` as container for in-app **notifications** (we don't use/follow material spec)
We also use overlay for different use-cases like `full-window` dialogs, custom `drop-downs` , and such things
opening new overlay is hiding the **notifications** which we do not want

The issue names no files or tests. Start by locating the Angular CDK overlay implementation and its existing tests, then trace how overlay stacking order is assigned. Done should include a supported way to keep a marked overlay above other overlays, with tests covering creation order and the notification use case.

Written by the indexing model from the issue text.